The present invention relates to a method for controlling the operation of changing to a higher gear (upshift) in a motor vehicle equipped with a double clutch transmission.
With reference to FIG. 1 of the accompanying drawings, a typical double clutch transmission for a motor vehicle is generally indicated 10 and comprises a first primary shaft 12 carrying a plurality of driving gearwheels (only one of which is schematically shown and is indicated 14) associated with a first set of gears (for example the odd gears), a second primary shaft 16 carrying a plurality of driving gearwheels (only one of which is schematically shown and is indicated 18) associated with the remaining gears (for example the even gears), a first friction clutch 20 designed to couple the first primary shaft 12 for rotation with a driving shaft 22 of the vehicle, a second friction clutch 24 designed to couple the second primary shaft 16 for rotation with the driving shaft 22, and one or more secondary shafts (in the example shown a first secondary shaft 26 and a second secondary shaft 28) carrying a plurality of driven gearwheels (of which only a first driven gearwheel 30 is shown schematically, said gearwheel being carried by the first secondary shaft 26 and meshing permanently with the driving gearwheel 14, together with a second driven gearwheel 32, which is carried by the second secondary shaft 28 and meshes permanently with the driving gearwheel 18) each associated with a respective gear and respective final reduction pinions 34 and 36 permanently meshing with a gearwheel 38 of the differential for transmission of the torque to the drive shafts 40 of the motor vehicle, and therefore to the driving wheels 42. The transmission of the torque may therefore take place both via a first section of the transmission consisting of the first friction clutch 20, the first primary shaft 12 and the first secondary shaft 26, and via a second section of the transmission consisting of the second friction clutch 24, the second primary shaft 16 and the second secondary shaft 28.
In a transmission of this type, the gear changing operation is performed in so-called power shift mode, namely with a phase where the starting gears (disengaging gear) and the end gears (engaging gear), which are each associated with a respective section of the transmission, are simultaneously engaged and transmission of the torque takes place via both sections of the transmission. Transfer of the torque from the disengaging section to the engaging section of the transmission therefore is performed by opening the friction clutch associated with the disengaging gear and simultaneously closing the friction clutch associated with the engaging gear. This phase is referred to herein below as the crossover phase of the friction clutches, or simply as the crossover phase. Opening of a clutch and simultaneous closing of the other clutch must be modulated according to suitable control logics which aim to ensure that the gear changing operation is performed as rapidly and comfortably as possible.
A gear changing operation will be described now in detail, assuming changing from a lower gear associated with the first section of the transmission to a higher gear associated with the second section of the transmission, considering a rigid axle model of the transmission of FIG. 1 and representing the involved parameters and variables with the symbols indicated in the following table.
SymbolDescription of parameter/variableCmtorque transmitted by the engineCF,distorque transmitted by the friction clutch associated with thedisengaging gearCF,inntorque transmitted by the friction clutch associated with theengaging gearCRresistive torque acting on the wheels (including gradient)ωmangular velocity of the driving shaftωp,disangular velocity of the primary shaft associated with thedisengaging gearωp,innangular velocity of the primary shaft associated with theengaging gearωRangular velocity of the wheelsMmass of the wheelsRradius of the wheelsJmmoment of inertia of the engineJp,disequivalent moment of inertia of the transmission sectionassociated with the disengaging gearJp,innequivalent moment of inertia of the transmission sectionassociated with the engaging gearJRoverall moment of inertia of the four wheels of the vehicleJeqequivalent moment of inertia at the wheelsτdistransmission ratio (including axle ratio) on the transmissionsection associated with the disengaging gearτinntransmission ratio (including axle ratio) on the transmissionsection associated with the engaging gear
The crossover phase of the friction clutches is preceded by a phase where the transmission of the driving power from the engine to the wheels occurs via a single section of the transmission, namely the section associated with the disengaging gear, the first friction clutch 20 (disengaging gear) being closed and the second friction clutch 24 (engaging gear) being open.
The simplified model represented by the following equation therefore applies:
                                                        (                                                J                  m                                +                                  J                                      p                    ,                    dis                                                              )                        ·                                          ω                .                            m                                =                                    C              m                        -                                          C                R                            τ                                      ,                            (        1        )            where the angular velocity ωm of the driving shaft 22 is synchronized with the angular velocity ωp,dis of the first primary shaft 12 (disengaging gear) and where the resistive torque CR is considered to be constant for the entire gear changing operation.
The equivalent moment of inertia, referred to the primary shaft, of the transmission section associated with the disengaging gear is defined, without taking into account the moments of inertia of the individual shafts of the transmission, by the following equation:
                              J                      p            ,            dis                          =                                            J              eq                                      τ              2                                =                                                                      J                  R                                +                                  M                  ·                                      R                    2                                                                              τ                2                                      .                                              (        2        )            
The gear changing operation provides firstly for engagement of the engaging gear, obtained by means of rotational coupling between the idle gearwheel (which may be equally well the driving gearwheel or the driven gearwheel) of the gearing associated with this gear and the respective shaft (primary or secondary shaft, respectively), typically by means of a sliding engaging sleeve, while the friction clutch associated with the engaging gear is kept open, then the crossover phase of the friction clutches, during which the friction clutch associated with the disengaging gear is gradually opened, while the friction clutch associated with the engaging gear is gradually closed, and finally disengagement of the disengaging gear by means of uncoupling of the idle gearwheel (driving or driven gearwheel) of the gearing associated with this gear from the respective shaft (primary or secondary shaft, respectively).
The simplified model which describes the crossover phase, during which both the clutches are in a slipping condition, is represented by the following equations:
                                                        J              m                        ·                                          ω                .                            m                                =                                    C              m                        -                          C                              F                ,                inn                                      -                          C                              F                ,                dis                                                    ;                            (        3        )                                                                    J              eq                        ·                                          ω                .                            R                                =                                                    τ                inn                            ·                              C                                  F                  ,                  inn                                                      +                                          τ                dis                            ·                              C                                  F                  ,                  dis                                                      -                          C              R                                      ;                            (        4        )                                          ω          R                =                                            ω                              p                ,                inn                                                    τ              inn                                =                                                    ω                                  p                  ,                  dis                                                            τ                dis                                      .                                              (        5        )            
A first constraint imposed in the known strategies for controlling the gear changing operation is the synchronism between the angular velocities ωm of the driving shaft 22 and ωp,dis of the first primary shaft 12 (disengaging gear). The following relation must therefore apply:ωm=ωp,dis.  (6)
Assuming that the angular velocities ωm and ωp,dis are the same at the start time of the gear changing operation, the synchronism between the angular velocities of the driving shaft 22 and of the first primary shaft 12 (disengaging gear) during the entire crossover phase is ensured if the following relation is satisfied:{dot over (ω)}m(t)={dot over (ω)}p,dis(t)  (7)
Since ωp,dis and ωR are linked, on the basis of the equation (5), by the following relation:ωp,dis=τdis·ωR,  (8)and, if the abovementioned condition of synchronism of the angular accelerations expressed by the relation (7) is set, the equation (3) becomes:Jm·τdis·{dot over (ω)}R=Cm−CF,inn−CF,dis.  (9)Taking {dot over (ω)}R from the equation (4) and substituting it in the equation (9), the following equation which links the torque profiles of the engine and of the two friction clutches 20 and 24 in the condition of synchronism between driving shaft 22 and first primary shaft 12 (disengaging gear) is obtained:Jeq·Cm−(Jeq+Jm·τinn·τdis)·CF,inn−CF,dis·(Jeq+Jm·τdis2)+Jm·τdis·CR=0.  (10)
Taking CF,dis from the equation (10), the minimum torque profile of the first friction clutch 20 which ensures synchronism between the angular velocities ωm and ωp,dis of the driving shaft 22 and of the first primary shaft 12 (disengaging gear), respectively, is obtained:
                                          C            _                                F            ,            dis                          =                                            1                              1                +                                                                            J                      m                                        ·                                          τ                      dis                      2                                                                            J                    eq                                                                        ·                          C              m                                -                                                                      J                  eq                                +                                                      J                    m                                    ·                                      τ                    dis                                    ·                                      τ                    inn                                                                                                J                  eq                                +                                                      J                    m                                    ·                                      τ                    dis                    2                                                                        ·                          C                              F                ,                inn                                              +                                    1                                                                    J                    eq                                                                              J                      m                                        ⁢                                          τ                      dis                                                                      +                                  τ                  dis                                                      ·                                          C                R                            .                                                          (        11        )            
If the first friction clutch 20 (disengaging gear) is controlled with a torque profile greater than the minimum torque profile defined by the equation (11), the previous transmission model with both the friction clutches 20 and 24 in a slipping condition, defined by the equations (3) and (4), is replaced by a new transmission model in which only the second friction clutch 24 is in a slipping condition, while the first friction clutch 20 is closed and therefore the first primary shaft 12 (disengaging gear) rotates at the same angular velocity as the driving shaft 22. This new model is defined by the following equations:
                                          ω            m                    =                                    ω                              p                ,                dis                                      =                                                                                τ                    dis                                                        τ                    inn                                                  ·                                  ω                                      p                    ,                    inn                                                              =                                                τ                  dis                                ·                                  ω                  R                                                                    ;                            (        12        )                                                      (                                                            J                  m                                ·                                  τ                  dis                  2                                            +                              J                R                                      )                    ·                                    ω              .                        R                          =                                            (                                                τ                  inn                                -                                  τ                  dis                                            )                        ·                          C                              F                ,                inn                                              +                                    τ              dis                        ·                          C              m                                -                                    C              R                        .                                              (        13        )            
If the open condition of the second friction clutch 24 is set at the beginning of the crossover phase (time t=0), i.e.CF,inn(0)=0,  (14)the initial minimum value of CF,dis:
                                                        C              _                                      F              ,              dis                                ⁡                      (            0            )                          =                                            1                              1                +                                                                            J                      m                                        ·                                          τ                      dis                      2                                                                            J                    eq                                                                        ·                                          C                m                            ⁡                              (                0                )                                              +                                    1                                                                    J                    eq                                                                              J                      m                                        ⁢                                          τ                      dis                                                                      +                                  τ                  dis                                                      ·                                          C                R                            .                                                          (        15        )            is obtained from the equation (11).
If, moreover, the open condition of the first friction clutch is set at the end of the crossover phase (instant t=tfi), i.e.CF,dis(tfi)=0,  (16)the following equation, which links the final value of the engine torque Cm to the final value of the torque CF,inn of the second friction clutch 24 (engaging gear):
                                          C            m                    ⁡                      (                          t              fi                        )                          =                                            (                              1                +                                                                            J                      m                                        ·                                          τ                      inn                                        ·                                          τ                      dis                                                                            J                    eq                                                              )                        ·                                          C                                  F                  ,                  inn                                            ⁡                              (                                  t                  fi                                )                                              -                                                                      J                  m                                ⁢                                  τ                  dis                                                            J                eq                                      ·                                          C                R                            .                                                          (        17        )            is obtained from the equation (10).
From the equation (17) it can be seen that, in the case of negligible resistive torques CR, the final torque Cm(tfi) of the engine is greater than the final torque CF,inn(tfi) of the second friction clutch 24 (engaging gear).
In short, the equation (15) represents a general constraint for selection of the torque profile of the first friction clutch (clutch to be opened) which ensures no slipping of this clutch, while the equation (17) represents the link between the final values of the engine torque and of the torque of the second friction clutch (clutch to be closed) which ensures the synchronism between the angular velocities of the driving shaft and of the first primary shaft (disengaging gear) and the continuity of the angular acceleration at the time of opening of the first friction clutch.